The present invention relates to permeation barriers for cells and, more particularly, to novel barriers for preventing entry of potentially deleterious vapors into the interior of display cells.
Cells, such as liquid crystal display cells, typically have a material, such as a liquid crystal composition and the like, contained between a pair of substrates, which may be formed of glass and the like materials in display cells, and with the periphery between the pair of spaced-apart substrates sealed by some material preventing egress of the internally-contained (liquid crystal) material from the cell. The sealing material must not only effectively seal the internal material within the cell but must also prevent entry into the cell of external materials which may have a deleterious effect upon the material contained within the cell. Typically, in liquid crystal display cells, the liquid crystal material is adversely affected by entry of water vapor. Accordingly, the material used for sealing the cell must not only keep the liquid crystal material within the cell, but must also prevent permeation of water vapor into the cell.
In the past, liquid crystal cell seals were formed either of glass frit or organic polymers, such as epoxy resins and the like. Sealing materials were generally disposed, in the form of gaskets, about the periphery of the cell. The advantage of a seal of glass frit is known to be that such seal is practically impervious to gas and vapors, but does require formation by high temperature processing which tend to distort the substrates and render difficult, if not impossible, adequately control of the uniformity of the distance between the inner surface of the parallel substrates, which gap (containing the liquid crystal material) must be maintained with a high degree of uniformity for proper operation of a liquid crystal display device. In producing an effective glass frit seal, the frit is generally applied to a surface of one of the substrates as a paste of glass powder particles dispersed in a liquid vehicle. The substrate is subsequently heated over a programmed temperature regime wherein, at lower temperatures, the solvent is evaporated and the binder is burned off, and thence in the higher temperature portions of the regime, the glass powder itself melts and coalesces to form a strongly adhesive bond to the glass substrate. Subsequently, the second glass substrate is positioned over the coalesced frit and the entire assembly is again subjected to a programmed temperature regime during which the temperature is raised within a few tens of degrees of the glazing temperature of the glass frit. At this relatively high temperature, the glass frit wets the second substrate to acquire satisfactory adhesion thereto. It is known that this second heating cycle tends to soften the substrates and cause warpage thereof, with the result that cells, particularly those of large surface area, sealed by this glass frit method tend to have a very low percentage of acceptable manufacture.
It is also known that warpage can be prevented by the use of organic polymer sealants, such as epoxy resins and the like, which are processible at much lower temperatures. Polymer sealants may be screen printed from a solution or dispersion of the polymer in a solvent, or a polymer sheet can be cut into the shape of a gasket which is sandwiched between the substrates to be sealed, and the sandwich is subsequently heated to effect such seal. It is also known to introduce the polymer along the edges of an assembly of two substrates which are kept otherwise separated by interior spacers. However, such organic polymer sealants have a relatively high permeability to water vapor. Under high humidity conditions, water vapor permeates through the organic polymer seal and tends to cause degradation of the liquid crystal material, typically by loss of molecular alignment, with eventual failure of a display device.
In accordance with the foregoing, it is desirable to provide a seal, and method of use thereof, for retaining material between a pair of spaced substrates, as in a liquid crystal display and the like, while providing an extremely low rate of permeation of vapor from the environment external to the cell into the contained material.